TEMPERATURE VARIATIONS ALONG STRATIGRAPHIC HEIGHT ACROSS THE BUSHVELD COMPLEX WITH IMPLICATIONS FOR MAGMA CHAMBER PROCESSES IN LAYERED INTRUSIONS

Systematic studies on major, minor, and trace elements and isotopes across the Bushveld Complex have revealed complicated magma chamber processes involving magma-wall rock interaction, magma recharge and fractional crystallization. However, the details of magma chamber processes for the Bushveld Complex are still poorly understood. To shed new light on the magma chamber processes in the Bushveld and layered intrusions in general, we calculate (near) crystallization temperatures for samples from the Bushveld Complex using a REE-in-plagioclase-pyroxene thermometer. This new thermometer is based on the plagioclase-pyroxene REE partitioning, and is derived from parameterized lattice strain models for pyroxene-melt and plagioclase-melt REE partitioning that we independently calibrated using published data. Due to the slow diffusion rates of REE in plagioclase and pyroxene, the REE-in-plagioclase-pyroxene thermometer can potentially record thermal events more close to magmatic temperatures for mafic and ultramafic rocks during fast cooling.

Application of the new thermometer to samples from the Bushveld Complex with published mineral compositions reveals systematic temperature variations along stratigraphic height and a strong correlation between the temperatures and initial 87Sr/86Sr in bulk samples. In the Lower and Critical zones, temperatures show a positive correlation with initial 87Sr/86Sr, suggesting that the magma chamber process was dominated by magma-wall rock assimilation and fractional crystallization with possibly increasing magma influx in the lower critical zone. In the Main and Upper zones, temperatures vary with nearly constant values of the initial 87Sr/86Sr, suggesting that the magma chambers were controlled largely by fractional crystallization and crystal settling. From the bottom to the top of the pyroxenite marker, the temperature shifts up ~170ºC with the initial 87Sr/86Sr decreasing from 0.708 to 0.707, consistent with large influx of new magmas. These initial results are encouraging and demand more detailed studies with higher spatial resolution along stratigraphic height. Variations in (near) magmatic temperatures and their correlations with chemical fractionation indices may offer new insights into the magma chamber processes in layered intrusions.